Taxonomic Discoveries: My Version of the Butterfly Effect

Witnessing a giant swallowtail (Papilio cresphontes) in flight is an incredible experience. It is the largest species of butterfly found in the US and Canada and with its yellow and black wings, it is impossible not to take pause and watch it flutter around the canopy. I will never forget the first time I saw one as a child. It was one of those moments that solidified my obsession with the natural world. Fast forward a few decades and now I can't help but ponder what kind of gardening I would need to do to attract these incredible insects to my yard. What I discovered surprised me to say the least. I had to plant something in the citrus family. 

We are all familiar with the fruits of various Rutaceae. This family contains the genus Citrus, providing humanity with oranges (C. × sinensis), lemons (C. × limon), grapefruits (C. × paradisi), and limes (mostly C. aurantifolia). These are largely tropical and subtropical trees, struggling to hang on anywhere temperatures dip below freezing regularly. How on Earth was a butterfly whose larva specialize on this family flitting around in temperate North America? What's more, reports place this species as far north as southern Quebec. I was obviously out of the loop on the taxonomic affinities of this family.

A little detective work turned up some surprising results. Temperate North America does in fact have some representatives of the citrus family. They are a far cry from an orange tree but they are nonetheless relatives. This inquiry actually solved a bit of trouble I was having with some riparian trees in my neck of the woods. As some of you probably know, trees are not a strong point of mine. I had encountered a few small woody things with compound leaves of three and dense clusters of greenish flowers. At first I thought I had found a rather robust poison ivy specimen but closer inspection revealed that wasn't the case.

Instead I had stumbled across something new for me - a common hoptree (Ptelea trifoliata). This cool looking tree is one of the giant swallowtails larval host trees, making it a member of -(you guessed it)- the citrus family. More often this small tree grows like a shrub with its tangle of multiple branches but they can reach some impressive heights, relatively speaking of course. Trees topping out at a height of 5 meters are not unheard of. Another common name of this tree - wafer ash - hints at its superficial similarity to a Fraxinus. Its compound leaves and wafer-like samaras are a bit of a curve ball for northerners like myself. It has a rather wide and patchy distribution throughout North America, and many subspecies/varieties have been named.

Common Hoptree ( Ptelea trifoliata  )

Common Hoptree (Ptelea trifoliata )

The other bit of this taxonomic journey involves another small tree, although this time I was better acquainted. Another host for the giant swallowtail is the prickly ash (Zanthoxylum americanum). It is interesting to note that both of these northern host trees superficially resemble ashes but I digress. The prickly ash is also small in stature and is most often found in thickets consisting of its own kind. As its common name suggests, you wouldn't want to go barreling through said thickets unless you wanted to donate some blood. It is well defended by sharp prickles on its stems. It does produce fruit but they are rather small and berry-like (technically follicles) and are distributed far and wide by birds.

Prickly Ash (  Zanthoxylum americanum   )

Prickly Ash ( Zanthoxylum americanum )

Both trees are rather aromatic. They produce volatile oily compounds like most of the family, making them smell quite pleasant. Their small size makes them interesting specimen trees for anyone looking for something unique to put in a native landscape. What's more, they host a variety of other larvae as well, including those of the spicebush swallowtail butterfly (P. troilus).

Together, these two species are the most northerly representatives of the citrus family, making them quite special indeed. I am happy that my interest in attracting giant swallowtails to my property resulted in a fascinating dive into the geography of this interesting family.

Photo Credits: [1] [2] [3]

Further Reading: [1] [2]

The Gas Plant

Meet the gas plant, Dictamnus albus. This lovely herbaceous species is native to southern Europe, north Africa, and Asia. The gas plant is a member of the citrus family, Rutaceae, and like many members of this group, it has very showy blossoms. Its affiliation with the citrus fruits on your counter isn't the only interesting thing about this species. As the common name might suggest, this plant does something quite strange. 

During the heat of summer, parts of the gas plant exude an oily substance that smells much like the fruits of its cousin, the lemon. These oils have been known to cause contact dermatitis not unlike the sap of giant hogweed. However, this is not the strangest aspect of the gas plants oily nature. One of the properties of these oils is that they are highly volatile. So volatile in fact that they can ignite. 

Another common name for this species is burning bush (though not the one of biblical lore). If air temperatures get high enough or if someone takes a match to this plant on a hot day, the oils covering its tissues will ignite in a flash. The oils burn off so quickly that it is of no consequence to the plant. It goes on growing like nothing ever happened. If you're like me then you have one burning question after reading this - why?!

Despite how incredible this phenomenon may seem, it doesn't appear that too many people have looked into its function. Research has identified a highly flammable organic compound within the oils called isoprene. In plants, isoprenes are thought to protect against heat stress. This is bolstered by the fact that the gas plant produces these compounds during the heat of summer. 

Another possibility is that spontaneous ignition of these compounds could create small wildfires that clear the surrounding area of competition. I have not seen any evidence suggesting this. Yet another possibility is that this is simply an unrelated side effect of oil production. Since the plant is not hurt by the quick burst of flames, it simply hasn't had any reason to evolve a less flammable alternative. Evolution is funny like that. 

Still don't believe what you are reading? Check out this video:

Photo Credit: Jörg Hempel (Wikimedia Commons)

Further Reading: [1]